1. Field of the Invention
The present invention relates to a novel adhesive composition and process for forming styrenic based adhesive and cement coatings on thermoplastic polymer surfaces, particularly to toy articles formed of elastomeric materials. Specifically, the present invention relates to hot melt adhesives comprising a high molecular weight polystyrene-ethylene/butylene-polystyrene block copolymer and articles constructed therefrom. These compositions are useful in a variety of hot melt adhesive applications such as children""s toys where adhesive composition superior particularly in high bonding strength, heat resistance, and hot-cold cycle resistance properties all particularly suited for unfriendly toy play environments.
2. Description of the Related Art
Children""s toys and dolls, in particular, have a history dating back into antiquity. Generally, the technology surrounding the manufacture of dolls centers on creation of an attractive doll face, miniturized application of conventional clothes, manufacture of a doll body using plush or other sculptural techniques, and the simulation of the hair with sculptural elements, fibers, or other techniques. In the case of doll faces, the doll maker has a wide panoply of possible constructions and manufacturing techniques at his disposal. Traditional techniques involve the use of plaster-like or hard plastics material to cast a doll face. In time, however, soft rubber materials such as styrene-butadiene rubber (SBR) and styrene-butadiene-styrene block copolymer (Kraton) elastomers have come to replace plaster and hard plastics in the production of doll heads because of their realistic look and feel.
The soft rubber doll faces include all the normal features of a human figure simulated by the doll including the entire head, including well formed lips, teeth, cheeks, nose, chin, ears, and forehead. After the injection molding of an elastomeric doll face, it is finished by applying a dye or other similar material to give the face a natural skin color. Thereafter the soft rubbery doll""s head must be joined to a body portion comprised of a torso and limbs to complete a full bodied toy doll item. This is generally accomplished by gluing, sealing or cementing the soft eleastomeric head potion to the headless body section generally composed of a hard or firm plastic. In selecting an adhesive for the purpose of affixing the dolls"" head two considerations are important: (1) that the adhesive form a firm and effective bond and seal between the soft elastomer and the harder resin substrate surface and (2) that the adhesive composition be relatively quick drying so as to accommodate expeditious doll manufacturing processes. The present invention provides a styrenic hot melt adhesive which provides superior bonding and rapid curing or drying.
Hot melt adhesives are recognized as adhesives which are tacky when applied in a molten or xe2x80x9cHot Meltxe2x80x9d state. These hot melt adhesives are typically solids and often at room temperature. Block copolymers have been used widely in the hot melt adhesive industry for a variety of applications. Block copolymers are often the preferred polymer base due to their good heat stability, high cohesive strength, and compatibility with a wide range of tackifiers and plasticizers. Kraton G-1651, a styrene-ethylene-butylene-styrene (S-EB-S) linear A-B-A block copolymer, having a molecular weight of about 240,000, was earlier intended for use by the injection molding industry for shoe soles. Due to its relatively high molecular weight, it was largely thought unsuitable for the hot melt adhesive industry, particularly as applied to consumer products such as toys.
Chen in U.S. Pat. No. 4,369,284 teaches dimensionally stable gelatinous elastomer compositions for use in connection with toys, therapeutic hand exercising grips, shock absorbers, acoustical insulators, and other uses. The essential ingredients of Chen consist of high styrene S-EB-S polymer in combination with high levels of plasticizing oil. Since such compositions do not include the use of tackifying resins, and that Chen teaches away from blending such polymers with other polymers, tackifiers, and fillers, such compositions are not useful as adhesives or high bonding cements and sealants generally required for toy items.
In U.S. Pat. No. 5,863,977 to Fischer et al. there is disclosed adhesive formulations using high molecular weight S-EB-S which are useful on products requiring adhesion to human skin properties (wound protective strips) and other adhesives which exhibit low self peel properties for release materials (peelable note paper). The formulations are suitable for such uses but inadequate for high bonding adhesive use in constructing durable use items such as toys and other materials subjected to heavy use or hostile environments.
Conventional block copolymers are very well known and disclosed in various patents such as Collins, U.S. Pat. No. 4,136,699, Malcolm et al., U.S. Pat. No. 5,057,571, Raykovitz et al., U.S. Pat. No. 4,704,110, Sieverding, U.S. Pat. No. 4,833,193, and Mostert, U.S. Pat. No. 4,937,138, all incorporated herein by reference in their entirety.
As indicated above, the class of the styrenic elastomeric materials commonly used as the molding material for toy doll faces and the harder structural portions of toy articles are A-B-A type block polymers such as styrene-butadiene-butylene copolymer-styrene or styrene-butadiene-styrene, manufactured by Shell and sold under the trademark Kraton. In the case of a toy having a hard surface comprised of a rigid or semirigid A-B-A styrene based block copolymer, compatible elastomeric A-B-A styrene copolymer in the form of styrene-ethylene-butylene-styrene, (SEBS, sold under the trademark Kraton-G) has been found to be an exceptional material for use as glues, cements, or sealants to bond mechanical parts in these A-B-A resin based toy items.
For adhesives used to bond toy mechanical parts and the like which are used in rugged or otherwise unfriendly environments it is considered necessary to have not only a high bond strength in normal state but also a high adhesive force to endure the heavy handling of child""s play. Moreover, the bonded mechanical parts must even be capable of enduring against extreme or sudden changes in temperature caused by leaving the toy item outdoors or immersing the toy item in hot and cold water, e.g. a child subjecting a doll to a sink bath. As indicated conventional prior art adhesive compositions are not satisfactory in meeting these demands as adhesives in toy items.
It has been found that high molecular weight A-B-A elastomeric block copolymers such as SEBS block copolymer and certain additives results in a hot melt adhesive which not only possesses high shear adhesive force, peeling bond strength and impact adhesive force but also exhibits an outstanding hot-cold cycle resistance and heat resistance, all ideally suited to application to children""s toy play articles.
It has been specifically discovered that A-B-A elastomeric block copolymers such as high molecular weight styrene-ethylene-butylene-styrene (SEBS) (sold as Kraton G-1651 by the Shell Chemical Co.) can be used as the base adhesive copolymer in a variety of hot melt applications where xe2x80x9cquick stickxe2x80x9d, high tack, flexibility and high cohesive high bond strength are important parameters. Furthermore, the applicants have found that incorporating SEBS along with another compatible A-B-A elastomeric copolymer in certain concentrations can dramatically improve the heat resistance, cold temperature flexibility, and cohesive bonding strength of a variety of adhesive formulations which are particularly useful in the toy industry.
In accordance with the present invention, there is provided an improved adhesive composition particularly useful for toy article applications comprising a hot melt adhesive containing:
(a) from about 50 percent to about 98 percent by weight of a base A-B-A block copolymer having a molecular weight greater than about 200,000, said block copolymer having polystyrene end blocks and a substantially saturated midblock;
(b) from about 1 (one) to 49% percent by weight of another A-B-A compatible elastomeric block copolymer;
(c) from about 0.50 to 5 percent of a liquid plasticizer; and
(d) from about 0.50 to 5 percent by weight of a solid tackifying agent,
wherein these basic adhesive ingredients total 100% by weight. The instant hot melt adhesive composition has a viscosity of about 2,000 to about 50,000 cP (centipoise) at 200 degrees C. and a melting point of about 150xc2x0 C. to about 200xc2x0 C. It is preferable that the block copolymer and the other A-B-A compatible elastomeric copolymer ingredients together comprise from about 90 to 98 weight percent of the total composition, the other second ABA compatible copolymer being present an amount of from about 20 to 35 weight percent being.
Adhesive compositions exhibiting high tack in combination with high cohesive strength are useful in a variety of applications. These adhesive formulations display unique adhesion to thermoplastic substrates in general and styrenic surfaces in particular forming strong cohesive bonding between adjoined surfaces of these materials particularly effective in the manufacture of hostile-use play toy items such as dolls. The instant hot melt adhesives are useful in application because they can be expeditiously spot applied by hot melt guns to affix or laminate fixtures in or to elastomeric substrate toy articles. Other applications for which the instant SEBS hot melt adhesive compositions are useful include any adjoined surfaces of articles comprised of thermoplastic resin materials such as internal automobile dashboard components, gaskets, outside surface repair kits, and other flexible attachment adhesive applications.
A preferred A-B-A copolymer of the present invention is high molecular weight styrene-ethylene-butylene-styrene (SEBS). The styrenic SEBS copolymer adhesive compositions made in accordance with the present invention are heated a rapid manner to enable a strong bond of the copolymer with polymeric substrate surfaces thereby engendering firm bonding of one surface with a substrate surface; that is, a sandwiched film of adhesive between an applied blanket cover to a substrate or one material surface or point affixed to another will afford an extremely strong bond between the two joined surfaces. The discovery of the present invention is best described as a mixture of polymeric additives, plasticizers, and high molecular SEBS which molecularly rearrange to produce excellent strong bond adhesive properties at high temperatures of up to 350xc2x0 C. and can be applied and manipulated at temperatures of from about 180xc2x0 C. to 200xc2x0 C. The use of SEBS in combination with the other compatible elastomeric polymer and tackifier resins allow adjoined or laminated surfaces to cohesively and strongly bond with one another with a necessary elasticity in the bonding or lamination for most applications, especially toy manufacturing. This adhesive composition is optimally used for gluing soft flexible elastomeric webs or covers onto hard A-B-A styrenic copolymer substrate surfaces. It enables the flexibility necessary for gluing or working two elastomeric polymeric materials. The adhesive compositions herein have particular advantage and application to children""s toys because of the toy industry""s pervasive use of styrenic block soft and hard elastomeric materials where soft elastomeric materials are used in doll facial structures and the adhesion of these rubber facial structures to a hard resin torso and limb structure is expected to withstand the rigorous and hostile environment of child use.
In a further embodiment, the present invention is even more particularly directed to a adhesive coating composition for elastomeric substrates comprising:
(a) about 50 percent to about 98 percent by weight of an A-B-A block copolymer having a molecular weight greater than about 200,000, said block copolymer having polystyrene end blocks and a substantially saturated midblock copolymer and selected from the group comprising styrene butadiene rubber (SBR), styrene-butadiene-styrene (S-B-S), and styrene-ethylene-butylene styrene (SEBS) elastomeric block copolymers;
(b) from about 1 to 49 percent by weight of another A-B-A compatible block copolymer in which the B segment is selected from the group consisting essentially of polyethylene, polypropylene, polybutylene, isoprene, and combinations thereof (eg. S-EP-S);
(c) from about 0.5 to 5 wt percent of a compatible liquid plasticizer; and
(d) from about 0.5 to 5 percent by weight of a compatible solid tackifing agent.